Storage tube



Oct. 31, 1950 R. E. GRAHAM 2,527,632

STORAGE TUBE Filed Jan. 29, 1948 2 Sheets-Sheet 1 I? 352752- 29 30 32 5% --1-K-1-fi:

TH/N CONDUC TING 2/ SOURCE LA YERS SIGNALS ARRAY 0R 5/1557 0F DIAMOND PARTICLES 0R 0mm MATERIAL EXHIBIT/N6 ELECTRON BOMBARDMENT //voucao CONDUCflV/TV l3 l0 I6 24 25 26 I2 2 FIG. 3 I4 :1 r

--r OUTPUT //v VENTOR R. E. GRAHAM AT ORNEV Oct. 31, 1950 2 Sheets-Sheet 2 Filed Jan. 29, 1948 FIG. 4

N 0 mm 5 m u T r 7 lxilllllalllill llil 5 3* B W m 5 A 5 m s a E m mm t E! 2 T A m W. E H mm mm M 05 U W E0 TU TH E MC sT 3m M H v r! c llll. 6 llllllllslllllllllll 5 1 r 0 M1. 2 EMA Wm 0 w. 0L sm INVENTOR 5. GRAHAM Patented Oct. 31, 1950 UNITED STATES PATENT OFFICE STORAGE TUBE Robert E. Graham, New York, N. Y., assignor to Bell Telephone Laboratories, Incorporated, New York, N. Y., a corporation of New York Application January .29, 1948, Serial No. 5,010

9 Claims. (01. 250164) This invention relates to storage devices and more particularly to devices of. this character utilizing electron beams for storing and reproduce ing electrical signals.

It is an object of this invention to utilize in storage tubes of the electron beam type materials exhibiting the property of electron bombardment induced conductivity.

It is another object of this invention to reduce, in storage tubes of this type, the distortions produced by stray secondary electrons.

In the copending applications of D. E. Wooldridge, Serial .No. 747,888, filed May 14, 1947, and K. G. McKay, Serial No. 789,667, filed December 4, 1947, there are disclosed various materials which exhibit the property known as bombardment induced conductivity. Each of these materials (such as, for example, diamond, zinc sulphide, magnesium oxide, silicon carbide and stibnite) is normally an insulator but when it is struck by electrons (or other particles, such as alpha or beta particles, for example) it becomes conductinif at the time an electric field exists between opposite surfaces of the insulator. The bombarding particles penetrate the insulator, causing a disruptive separation of the positive and negative charges specific to the atoms which are affected by the bombarding particles. These charges are drawn toward the electrodes producing the electric field and this motion of charges constitutes a conduction current which is in many cases greatly in excess of the current of the bombarding particles.

Diamond is a favored solid insulator for this work (although other materials such as, for example, others mentioned in the Wooldridgeand McKay applications can be used) because it can easily be obtained without suificient impurities o-r imperfections to affect its high insulation resistance or its conducting properties under bombardment. The carbon atoms therein consist each of a nucleus exhibiting fixed units of positive charge,

to which two electrons are tightly bound. This core is surrounded by four valence electrons,

The carbon atoms are held together by electron pair bonds between adjacent atoms. The 'insulation resistance is high because the electron bonds are very tight. As a result of this tightness, very few electrons are displaced from their bonds by thermal agitation. This is not the case in, for example, metals, where a'large number of electrons are continuously being displaced by thermal agitation and are relatively free to wander through the'metal this, under normal con-- ditions, constituting the usual current in a metal; conducting medium.

When electron bombardment removes a valence electron from its bonds in an insulating target, producing a deficiency of one electron in the atomic structure immediately affected, this localized electron deficiency is called a hole. Under an applied electric field the arrangement of the electrons is changed, and the location-of any given hole will change. As a consequence,- the hole can be conveniently regarded as a positive particle which is free to move under the. influence of the field. Similarly, the electron freed from the bond in question constitutes a negative particle which is free to move under the influence of the electric field. If there is no applied field, any free electron or positive hole moves in accordance with thermal agitation and consequently has a completely random motion. Under an applied electric field, there is a directional motion superimposed on the random one. The order of mobility of the electrons in diamond is of the order of 1,000 centimeters per secondfor. a field of one volt .per centimeter. For a fieldof. 10% volts per centimeter the velocity therefore is 10' centimeters-per second. Fora diamond crystal one millimeter thick the transit time therefore is 10- seconds. The mobility ofthe elect-J trons is affected bythe number of traps,. that is, the presence of foreign atoms or imperfections in the crystal. If an electron gets into a trap, it takes a greater or less amount of time to get out, depending upon the thermal energy re-:

quired. Further information on traps and other characteristics of diamond crystals is given in the Wooldridge and McKay applications re-':

ferred to above.

. In accordance with the present invention, thereis provided a storage tube of theelectron beam type including an electron target embodying ma;

terial which exhibits the property of electron bombardment induced conductivity. Diamond tween two thin metal coatings. The signal to be stored is applied as a voltage between the twometal coatings (preferably in series with a constant polarizing voltage) while orle of the metal coatings is scanned by a beam of li'igh velocity electrons. The signal is reproduced 'by' removing the voltage between the coatings an'd: rescanning with the beam, the reproduced signal being formed as a voltage across an output resistor.

In a modification, the signal to be stored is utilized to modulate the scanning beam, a constant polarizing voltage being applied across the layer of material exhibiting electron bombardment induced conductivity. To recover the signal, the direct current polarizin voltage is removed and the target is scanned with a constant intensity beam.

The invention will be more readily understood by referring to the following description taken in connection with the accompanying drawing forming a part thereof in which:

Fig. 1 is a schematic representation of a cathode ray storage tube of this invention together with certain of its associated circuits and auxiliary equipment;

Fig. 2 is a schematic vie showing, in greatly enlarged form, a portion of the target of the tube shown in Fig. 1;

Fig. 3 shows a modification of the device shown in Fig. 1 and Fig. 4 is a schematic circuit diagram utilizing two of the devices of Fig. 1.

Referring more particularly to the drawings, Fig. 1 shows, by way of example to illustrate the invention, a cathode ray storage tube I employing a target II containing material exhibiting the property of electron bombardment induced conductivity. The tube I0 comprises an evacuated container I2 enclosing the target I I, an electron gun I3 for generating, focussing and accelerating a beam of high velocity electrons toward this target, and two sets of electrostatic defiecting plates I4 and I5 for causing the beam of electrons to scan, for example, every elemental area in turn of a desired field on the target II. The target II, which will be described below in greater detail with reference to Fig. 2, comprises a layer of material I6 which exhibits the property of electron bombardment induced conductivity sandwiched between two metallayers I? and I8. By means of a suitable switch I9, a source of signals can'be connected between the layers I1 and IS, a polarizing source of potential 2| also being included in this circuit, for the recording operation, and the source 20 removed and a signal utilization device 22 connected across the resistor 32 during the reproducing operation.

The electron gun I3 preferably comprises a,

cathode 23, a control electrode or member 24, a first anode member 25, and a second and final anode comprising a cylindrical member 26 and a coating 21 of conducting material on the inside Walls of the envelope I2 extending from the region of the cylinder 26 to the region of the target II. The control electrode 24 is placed at any suitable negative potential with respect to the potential of the cathode 23 by means of an adjustable source 28, and the first anode 25 and the final anode 26, 21 are placed at appropriate positive potentials with respect to the cathode 23 by means of the source 29 and the source 30. As an example, the final anode 26, 2'1 can be from 1,000 to 10,000 or more, volts positive with respect to the cathode and the first anode 25 can have an appropriate lower positive voltage for proper focussing. Any suitable source 3| can be utilized to heat the cathode 23. The negative terminal of the source 29 is connected to the cathode 23 and the positive terminal thereof is connected to the first anode 25, while the negative terminal of the source 30 is connected to the positive terminal of the source 29 and the positive terminal of source 39 is connected to the second anode 26, 21 and to ground. Batteries have been shown only for convenience in the drawing and it is to be understood that any other means for producing direct voltages can be used instead.

Reference will now be made to Fig. 2 which shows in enlarged scale a portion of the target II. Fig. 2 has not been drawn to scale. As mentioned above, the target II comprises a thin layer I6 of insulating material which exhibits the property of electron bombardment induced conductivity and which is sandwiched between two thin metal layers I1 and I8. By way of example, the layer I5 is a very thin out of diamond or a simulated sheet of diamond formed by a crystalline layer (preferably on particle thick) of diamond chips or diamond dust. Alternatively, the layer 2'! can be of any other suitable material exhibiting this desired property. The layer I6. can be of the order of a millimeter thick, for example. The layers I 'I and I8 are of any suitable material such as gold, silver, platinum or aluminum, for example.

The layer I1 is connected directly to ground while the layer I8 is connected to the switch I9. The upper contact of the switch member I9 is connected to the signal utilization device 22 while the lower contact thereof is connected through the biasing source of potential H to the source of signals 20. When the switch I9 is in its lower position, signals are being recorded on the target I I of the tube I0 and when theswitch I9 is operated to its upper position (as by, for example, suitable commutating or controlling means) the reproduced signal is applied to the signal utilization device which may be, for example, an amplifier or an oscilloscope, or another tube like the device ID if it is desired to produce further delay, or any other suitable device utilizing reproduced signals.

The high velocity beam produced by the gun I3 is deflected over a desired area on the target by means of appropriate potentials applied to the deflecting plates I4 and I5 by electrostatic sweep circuits (not shown). As examples of satisfactory sweep circuits if the signal to be stored is a television or video signal corresponding to one complete frame, reference is made to Patent 2,178,464 issued October 31, 1939, to M. W. Baldwin, Jr., which discloses balanced electrostatic sweep circuits suitable for this purpose. Connections can be made from the balanced sweep circuits to the pairs of plates I4and I5 by means of coupling condensers 40, II, 42 and 43, respectively, of about one microfarad capacity each. Coupling resistances 44 and 45, of the order of many megohms each, are respectively connected across the pairs of plates l4 and I5. The midpoints of the resistances 44 and 45 are connected to the positive terminal of the source 30 so that the average of the potentials of the deflecting plates does not deviate more than slightly from the potential of the anode 26, 27. This relationship is maintained to avoid changes in the sensitivity of the deflecting systems and the consequent distortion of the image which would otherwise result. For a more complete description of the advantages of balanced sweep circuits for use with cathode ray television tubes, reference is made to the above-mentioned Baldwin patent and also to Patent 2,209,199 issued July 23, 1940, to Frank Gray. The sweep circuits have the desired circuit constants and are so synchronized as to produce the type of scanning desired.

The operation of the device shown in Fig. :1 will now be described. The signal to be stored which may be, for example, a television video signal corresponding to a complete frame, is applied between the metal layers I I and I8 positioned on opposite sides of the layer l6 which possesses the property of electron bombardment induced conductivity, the circuit starting with ground and passing through the source of signals 20, the polarizing source 2| (which may be polarized in either direction), switch [9 through its lower contact, metal layer l8, crystalline layer [6, and metal layer I! back to ground. With the switch member IS in its lower position, the source of signals is connected in the circuit while the signal utilization device 22 is disconnected therefrom. While the signals from the source 20 are applied across the member IS, the member I6 is scanned (through the metal coating IT) by the high velocity electron beam produced by the electron gun l3. The scanning beam bombards, an elemental area at a time, the material It through the metal coating l1 and induces conductivity in each elemental region in turn. The freed charges in the material l6 moves under the influence of the polarizing field set up by the instantaneous intensity of the signal to be recorded. During this elemental period, a fraction of the freed charges are trapped in characteristic imperfections in the crystal structure of the material I6, setting up a local field opposing the signal from the source 20. The concentration of charges and thus the strength of the stored field varies over the area being scanned according to the instantaneous values of the signal from the source 20 during the scan.

During the recovery operation, the switch I9 is moved to its upper position, connecting the output resistor 32 between the layer I8 and ground and disconnecting the source of signals 30 and the polarizing source 2| from layer l8, and the high velocity beam is again caused to scan the crystal layer l6 and set up point-bypoint conductivity between the two metal electrodes l1 and 18. The current which flows for any given beam position depends upon the polarizing field strength due to the local trapped charges. This current flow produces an instantaneous voltage drop across the resistor 32 corresponding to the signal strength at this element during the recording operation. The current fiow during the recovery period is in the opposite direction to that occurring during the storage interval and acts to neutralize the trapped charges. The flow of current through the resistor 32 produces a voltage drop which is applied to the signal utilization device 22.

Another way in which the signal can be stored is to use the signal to modulate the scanning beam current using a constant polarizing voltage across layer IB. Such an arrangement is shown in Fig. 3. The signal to be stored is applied between the terminals 50 and 51 to cause the instantaneous intensity of the beam to be varied in accordance with the corresponding signal strength, a steady polarizing source 52 being applied between metal layers l1 and I8. This causes a pattern of trapped charges to be set up in the layer I6. To recover the signal, the direct current polarizing voltage source 52 is removed by moving switch 54 from its first to its second position, this switch movement also removing the input signals leaving a steady beam control voltage. Thus the crystal layer I6 is time.

scanned with a steady beam and the reproduced (delayedl signal appears across the resistor 32 which can be connected to a signal utilization device 22, as in Fig. 1. Except for the differences above noted, the apparatus and the method of operation of Fig. 3 are the same as in the arrangement of Fig. l. v

An important advantage of the type of storage tube shown in Figs. 1 and 3 lies in its relative freedom from stray secondary emission troubles.

One application of the storage tube I0 is to delay a television or video signal by a frame time. For this purpose a pair of storage tubes IDA and 10B is used, each one being used alternately for storage and for reproduction, one being recording when the other is reproducing and vice versa. Fig. 4 illustrates this use for thestorage method of Fig. 1 (an analogous circuit can be used if the method of Fig. 3 is utilized). In either case, the synchronizing signals associated with the incoming signal are used to trigger off the sweep signals supplied to the deflection input terminals (see elements 40, 4|, 42, and 43 of Fig. 1 or Fig. 3) The sweep waves are of the conventional saw-tooth wave type which are used in receiving cathode-ray picture tubes. When storage tube [0A is being used for recording, as shown in Fig. 4, tube [0B is being used to pick up or reproduce the frame period of signals previously recorded thereon. At the end of each frame time, the functions of the two tubes [0A and IDB are interchanged. Any suitable commutator device 55, cooperating with switches 56 and 57, can be used to switch the tubes [0A and [0B after each frame Any well-known means for synchronizing the commutator device with the sweep circuits (not shown) can be used. If desired, the devices 55, 56 and 51 can be replaced by suitable electronic switching means.

Various other modifications can be made in the embodiments described above without departing from the spirit of the invention the scope of which is indicated in the claims. The specific potentials applied to the various elements are herein given merely by way of example and it is to be understood that their values may be made materially different without changing the general method of operation of the devices described herein. For example, the type of crystal used and its thickness materially affect the operating potentials required.

What is claimed is:

1. A device for the storage of electric signals comprising a target for electrons including an array of particles of an electrically insulating material which has the property of becoming electrically conducting when bombarded with electrons, conducting surfaces on respectively opposite sides of said array and in contact therewith, respective connections to said conducting surfaces, and means including a source of direct potential for applying a voltage between said respective connections.

2. A device for the storage of electrical signals comprising a target for electrons including material which is normally electrically insulating but which has the property of becoming electrically conducting when bombarded with electrons, and means including a source of direct potential for applying a signal voltage to be stored between two respectively opposite surfaces of said material.

3. A device for the storage of electrical signals comprising a target for electrons including material which is normally electrically insulating but which has the property of becoming electrically conducting when bombarded with electrons, electric circuit means for applying a signal voltage to be stored between two respectively opposite surfaces of said material, and means separate from said signal applying means for scanning said target with a beam of electrons.

4. A device for the storage of electrical signals comprising a target for electrons including a layer of electrically insulating material which has the property of becoming electrically conducting when bombarded with electrons, means including a source of direct potential for applying a voltage across said layer, and means for scanning said layer with a modulated beam of electrons.

5. A recorder and reproducer of signals comprising a target for electrons including a layer of an electrical insulator which possesses the property of becoming an electrical conductor when bombarded with electrons, electric circuit means for applying a signal to be stored to said layer, means for forming a beam of electrons and for causing it to strike said target as said signals are being applied thereto, thereby trapping a charge in said target proportional to the intensity of the signal, and means for utilizing said trapped charge at a later time to control the production of signals.

6. A tube for the storage of electrical signals comprising a target for electrons including material which is normally electrically insulating but which has the property of becoming electrically conducting when bombarded with electrons, two metal coatings on respectively opposite surfaces of said material, means for applying a signal voltage to be stored between said two metal coatings, and means for scanning said target with a beam of electrons.

7. A tube for the storage of electrical signals comprising a target for electrons including material which is normally electrically insulating but which has the property of becoming electrically conducting when bombarded with electrons, electric circuit means for applying a voltage between two respectively opposite surfaces of said material, and means separate from said electric circuit means for scanning said target with a modulated beam of electrons.

8. A tube for the storage of electrical signals comprising a target for electrons including material which is normally electrically insulating but which has the property of becoming electrically conducting when bombarded with electrons, two metal coatings on respectively opposite surfaces of said material, means for applying a voltage between said two coatings, and means for scanning said target with a beam of electrons.

9. A tube for the storage of electrical signals comprising a target for electrons including material which is normally electrically insulating but which has the property of becoming electrically conducting when bombarded with electrons, two metal coatings on respectively opposite surfaces of said material, means for applying a voltage between said two coatings, and means for scanning said target with a modulated beam of electrons.

ROBERT E. GRAHAM.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,124,057 Farnsworth July 19, 1938 2,277,008 Von Ardenne Mar. 17, 1942 2,277,009 Von Ardenne Mar. 17, 1942 2,460,093 Law Jan. 25, 1949 2,462,569 Sziklai Feb. 22, 1949 2,470,875 Snyder May 24, 1949 

